Electrical apparatus



; i 1 Fa Z l INVENTOR. BY W wi$fiam34own Sept. 21, 1943. c. w. BROWN 2,330,109

ELECTRICAL APPARATUS Filed March 6, 1942 2 Sheets-Sheet l ATTORNEY.

Sept. 21, 1943. c. w. BROWN ELECTRICAL APPARATUS Filed March 6, 1942 2 Sheets-Sheet 2 INVENOR.

ATTORNEX Patented 21, 1943 amomcn. maaarns Cyril William Brown, Wallington, England, noaignorto Radio Patents Corporation, a corporation of New York mutation March 0, 194:, Serial No. 433,020

., mom; Britain March 14 1941 This invention relates to arrangements for coupling theoutput end or a transmission line into a thermionic valve amplifier.

The principal object oi'the invention is to ie vide a means of terminating a transmission line at its output end in an impedance which offers a low resistance to alternating current and which- ,oflers a higher resistance to direct current.

In the arrangement accordingto. the inven tion the output end or the transmission line is connected as a cathode load common to the control grid and anode circuits 01' a valve, the control grid of this valve being maintained at substantially zero signal potential and the anode load impedance being of low value compared with the anode impedance of the valve.

By this arrangement the transmission line is.

terminated in a resistance to directcurrent equal to the direct current resistance oithe valve and it is terminated in a resistance to alternating current equal to. the reciprocal oi thesum oi (a) the mutual conductance of the valve and (b) the reciprocal oi the anode impedance of the valve. The reciprocal oithe anode impedance of the valve will generally be negligible in comparison with the mutual conductance oi the valve, so that the resistance to alternating current becomes equal to the reciprocal'oi the mutual conductance of the valve. f

If it is desired to avoid having di'rectcurrent ,7- as China. (or. 179-171) load, which'comprises resistances. and small series inductance ii to compensate torstray I shunt capacities, so that the resultant, cathode load is substantially purely resistive at all ire quencies within the frequency spectrum of the signalsto be transmitted. The transmission line.

comprising conductors 3, lfwhich may be parallel or concentric, is connected, acrosscathode load v2,, 2| and'carries the signals to the main amplifier, the first stage 01 which isthe circuit ofpentode valve 5. Line 4 is connected to the cathode of pentode 5 while line 3, being earth'ed at the input end, is maintained at zero signal potential. The control grid of pentode [is main-. tained at zero signal potential by couplingto linejjthrough condenser 6., The control grid bias is derived through grid leakresistance ,i irompotentiomete'r 8, which is connected across a high potentialsource'connected to line 3 at its circulating in the transmission line, or ii a resistance to alternating current lower than the reciprocal of the mutualconductanceoi the valve is required, a substantially purely, resistive impedance may be connected across the output end of the transmission line. "This further resistance will appear in parallel with both the direct current and the alternatin g current resistance values above mentioned. o

In the accompanying drawings, Figures 1 and '2 arecircuit diagramsshowing alternative arrangements for coupling oi the output end of 'a transmission lineinto thecircuit oi! a thermionic valve, these arrangements being suitable ior the transmission oi. television signals but without the direct current component'which represents average picture brightness. Figure 3 shows another arrangement, but in which the direct curnegative end. The screen grid of pentode 5 is maintained at the signal potential of the cathode by coupling condenser 9 and derives its supply potential from. potentiometer 8. .The suppressor grid is maintained at cathode potential by the conventional direct connection. Y

The anode load of pentode i'comprises resistance l0, and inseries therewith a small inductance l i to compensateior stray capacities. The output signals developed across anode load H), II are passed through coupling condenser l2 to output terminal l3. The anode current of pentode 6, circulates in transmission lines 3, 4, passing line 3, 4, is 100 ohms, it will be correctly matched rent. component of the signal is transmitted.

Figure 4 shows a push pull arrangement in which the direct current component is not transmitted.

In the arrangement shownin Figure-'1, the triode output valve l of a pre-amplifler oi tale;- vision signals is arranged as a'cathode iollower developing the output signals across its "cathode "so at the output end if the mutual conductance of pentode 5 is 10 milliamps. per volt,-.the anodeimpedance of pentode 5 being high in comparison with ohms.

In order to achieve the high mutual conductance of 10 milliamps. per volt, it may be necessary in practice for the pentode represented at 5 to be made up of two similar pentodes connected in parallel. t

II resistance 2 of, high value, the transmission line maysimilarly be matched at the input end by the employment for valve I of a valve, ,or a pair of valves in parallel, having a mutual conductance 01' I0 milliamps per volt. If, however, the anode impedance or the value of resistance-2 is comparable with the characteristic impedancedetermined by the values of resistance 1 and corn denser 8. 12 these are 1 megohm and 1 microiarad respectively, the time constantwill be 1 second.

In the arrangement illustrated in Figure across the output end of transmission line 3, 6 there is connected an impedance comprising s; resistance l4 and in ser'es therewith a small inductance ill for compensating for stray capaci-- ties. The input valve oi the main amplifier is here shown as a triode 20 instead of a pentode.

The control grid bias is shown as derived from grid leak resistance 22 from a, tapping on resistance it. As the anode impedance of a trlode is very much lower than that of a pentode, resistance Ill must be of low value and the inductance ll of Figure 1 will normally be unnecessary and is, therefore, omitted. It may be necessary to take into account the anode impedance of triode 20 as well as its mutual conductance for the purpose of calculating the input impedance presented to transmission line 3, t.

The value of resistance is may be selected in order to correct the matching between the transmission line and the resistance for alternating currents presented by the circuit of pentode ll. Alternatively, it may be selected with regard to the value oiresistance 2 so that no direct current will circulate through the transmission line 3, i. This will be achieved ii the direct current potential drop across resistance i i due to the anode current or valve 20 is equal to that across resistance 2 due to the anode current of valve i. If the anode current of valve l is milliamps, the value of resistance 2 may be 10,000 ohms. Assuming that the anode current of pentode B is milliamps, resistance i4 may then be 2,500 ohms,

so that the potential drops at both ends of trans- 4.

mission line 3, 4 will be volts.

In the arrangement shown in Figure 8. in or-.

der to transmit the direct current component of the signal, the control grid of pentode B is directly connected to a point on potentiometer 8i 5 which, in this case. should be of low resistance.

The part thereof between the control grid tapping and line 3 may, for example, be 500 ohms, so as to be of low value in comparison with the reactance due to grid-cathode capacity at the highest frequencies to be handled. The screen grid of pentode 5 is coupled by condenser 32 to the line 3 of zero signal potential. instead of to the cathode, because its 1). C. connection is to the point of low potential, not to the cathode. and it is desired that the circuit should respond in the same way to D. C. and 'A. C. signals. The eiiect of inserting the signal voltage from the transmission line between cathode and screen grid is small, because the screen grid has only a small mutual conductance. The output terminal I8 is directly connected to the anode instead or being conneeted through a coupling condenser. H

In the arrangement shown in Figure 4,; the output valves i, M or the pre-amplifier are connected in push-pull, and the input valve 6. 4B or the main amplifier are also inpush-pull. Neither of the conductors c, M of the transmission line is earthed, but the centre points or the cathode loads or both pairs of valves are earthed..

' The output circuits are similar to those of pentode 5 in Figure 1, and the push-pull output potential is developed between terminals i3, 43.

1 claim:

1. An electrical-system comprising a transmission line having high potential and low potential input and output terminals, a pair of vacuum tube amplifiers each having a cathode, a grid and a plate, a cathode load resistance common to the grid and plate circuits of the first tube for feeding the input of said line, the high potential input terminal of said line being connected to the cathode of the firsttube and the high potential output terminal or said line being connected to the cathode of the second tube, a connection for signal currents from thegrid of the second tube to the low potential output terminal of said line, whereby said line forms an input impedance common to the grid and plate circuits of said second tube and said resistance pr'ovides'a cathode return for the direct current of said second. tube, the reciprocal of the mutual conductance 0! each of said tubes substantially matching the characteristic impedance oi. said line, and a plate load resistance'ior said second tube. l

'2. An electrical system comprising a transmission line having high potential and low potential input and output terminals, a Pair of vacuum tube amplifiers each having a cathode, a grid and a plate, a cathode load resistance common to the grid and plate circuits of the first tube for feeding the input of said line, the high potential input terminal. of said. line being. connected to the cathode of the first tube and the high potential output terminaloi said line being connected to the cathode of the second tube, a capacitive circuit connection from the grid of said second tube to the low potential output terminaloi said line, whereby said line forms an'input impedance common to the grid and plate circuits of said second tube and said resistance provides a cathode return. for the direct current of said second tube.

the reciprocal of the mutual conductance of each of said tubes substantially matching the characteristic impedance of said line, and a plate load resistance for said second tube having a value low compared with the plate impedance of said second tube.

3. An electrical system comprising a transmission line having high potential and low potential input and output terminals, a pair of vacuum tube amplifiers each having a cathode, a grid and a plate, a cathode load resistance common to the grid and plate circuits of the first tube for feed.- ing the input of said line, the high potential input terminal of said line being connected to the cathode of the first tube and the. high potential output terminal of said line being connected to the cathode of the second.tube,..a connection for direct current from the grid of said second tube to the low potential output terminal 01 said line. whereby said line forms an input impedance common to the grid and plate circuits of said second tube and said resistance provides acathodereturn for the direct current of said second tube,

' value low compared with p 2;sso,1oe

. the reciprocal of the mutual conductance of each of said tubes substantially matching the characteristic impedance of said line, and a plate load resistance for said second tube having a value low compared with the plate impedance of said second tube. a s v 4. An electrical system comprising a transmission line having high potential and low potential input and output termin'alaa pair oi vacuum tube amplifiers each having a' cathode, agrid and a plate, a cathode load resistance having a value high in comparison to the characteristic impedance oisaid line and common to the grid and plate circuits of the first tube for feeding the input of saidiine. the high potential input terminal of said line being connected to the oathode oi the first tube and the high potential output terminal of said line being connected to the cathode of the second tube, a connection for signal currents irom the grid of saidsecond tube to the low potential output terminal of said line,

whereby said line iorms an input impedance com mon to the grid and plate circuits of said second tube and-said resistance providesa cathode re turn for the direct current or said second tube.

the reciprocal oi the mutual conductances of.

each oi. said tubes substantially matching the characteristic impedance 01' said line, and a plate load resistance .ior said second tube having a the plate impedance or said second tube.

5. An electrical system comprising a transmission line having high potentialand low potential input and output terminals,-a pair of vacuum type amplifiers each havingacathode, grid and plate.

a cathode load resistance having a value high compared with the characteristic impedance of said line and common tothe grid andplate cirsaid second tube, the plate impedance "oi said second tube being high compared with the characteristic impedance of saidline, a connection for signal currents from the grid of said second 5 tubeto the low potential output terminal or said line, whereby said line forms an input impedance common to the grid and plate circuits oi said second tube and said resistance provides a cathode return for the direct current of said second tube, the reciprocal of the mutual conductance of each of said tubes substantially matching the characteristic impedance of said line, and a plate load resistance'ior said second tube having a value small compared with the plate impedance of said second tube. g

6. In combination, a transmission line having high potential and low potential input and output terminals, a firstvacuum tube amplifier having at least a cathode grid and plate, a-second vacuum tube amplifler having at least a cathode, a control grid, a screen grid and a plate, a cathode load resistance having a value high compared with the characteristic impedance oi said line and common to the grid and plate circuits 0! said first tube for feeding the input 01' said line, the high'pote'ntial input terminal at said line'being connected to the cathode of the first tube and the high potential output terminal of said line being connected to the cathode of the second tube, a connection for signal current from the grid of said second tube to the low potential output terminal of said line, whereby said line forms an input impedance common-to the grid and plate circuits. oi said second tubeand said resistance" provides a cathode return for the direct current oi. said second tube the reciprocal or the mutual conductance of each at said. tubes substantially matching the characteristic impedance of said line, and a'plateload resistance ior said second- 40 tube havinga value low compared with the plate impedance of said second'tube.

, BROWN. 

